The trouble with psychological warfare is that it is difficult to measure its effects among all of the other tangible and intangible factors on a battlefield. Many times, we only discover its effectiveness long after the outcome of a battle, and even the war, is determined. Soldiers diaries are a good indicator as a primary source. Here are three examples:
The Rebel Yell
The Confederate Army during the American Civil War often found itself outnumbered on any given battlefield, so in an attempt to blunt that advantage, a tradition was begun whereby rebel soldiers would in unison scream when attacking. Scream is a poor descriptor, but we have no actual recordings of the rebel yell, of course, and by all accounts, it was no ordinary scream. Union soldiers who wrote of it later said that any soldier who heard the rebel yell and said they weren't afraid was lying.
Ju-87 "Stuka" Dive Bomber
The Germans during World War II perfected the use of precision close air support as part of their blitzkrieg style of warfare. The Ju-87 Stuka was an excellent aircraft for such missions. While the blitzkrieg was horrible and difficult enough to face on the battlefield, the Germans added a psychological warfare element to the Stuka as well. They attached an air-driven siren to the bottom of the aircraft so that, when it went into attack dives, the sirens went off with the attack. In squadrons attacking enemy lines in a battlefield, diary accounts said it made it seem as though every plane was headed straight for each individual soldier.
Vietcong POW Practices
In the jungles of South Vietnam, the Vietcong soldiers fighting there had little capacity to take prisoners, or to hold them for long periods of time. Rather than treating them as most nations and armies did (disarm,interrogate for intelligence information,hold as a bargaining chip), the VC would sometimes brutally torture, mutilate and kill Americans captured in battle, then hang the bodies where other American soldiers would discover them. The psychological effect provoked intense fear among young draftees, though it also had the effect of motivating them to fight harder to avoid capture.